Aside from heating element-stud welding techniques, professionals in the field use mostly heating coil welding to tightly join plastic components. In heating coil welding, heating coil fittings, also known as heating coil bushings, are used to axially join pipes or tubes, such as utility pipes for water or gas. For the welding process, heating coil fittings feature a synthetic pipe fitting, for example, made from polyethylene, with at least one exposed or concealed heating coil on or in the tube wall. The heating coil has two terminal contacts, which extend to the outside and may be connected with an electric welding power supply.
The two synthetic workpiece pipes are first inserted in a fitting tube with a slightly larger bore than the outer diameter of the workpiece pipes to be welded together. A welding voltage is then applied to the heating coil, a current passes through the heating coil and the so applied energy is transformed into heat in the electrical or ohmic resistance of the heating coil. Starting at the heating coil, the heat slowly permeates the synthetic materials of the pipes and fittings. These materials will plasticize and flow together, once they are past the softening point or when they have reached the melting range, respectively. Subsequent cooling yields a permanent homogenous bond between the two pipes and the fitting, which is gas- and waterproof. This entire process is called “heating coil welding” or also “welding”.
In an established heating coil welding method with an AC welding voltage feed from the welding power supply to the heating coil, the effective value for the welding voltage is kept constant throughout the welding process. Consequently, the required heat for the welding process is mainly achieved by adjusting the welding voltage and/or the welding time. Modern heating coil welders access the welding parameters via a data feed mechanism to set the welding voltage and time prior to the actual welding process.
In this process, the welding parameters may be imported via bar code. In order to exclude mistakes, the manufacturer may, for example, put a bar code label on each component, making the label a non-detachable, permanent feature of the fitting. This avoids common mistakes, such as input errors for the welding voltage and/or time.
The bar code design is standardized and contains not only the input parameters for the welding process. Providing the welder design accommodates these functions, the bar code also documents reference data for the laying of pipe networks, such as the manufacturer's information for the pipe components, where the pipes may be installed, etc. The bar codes also contain the known temperature compensation data, which the actual welding machine can use to adjust the appropriate welding energy input into the fitting.
In the past, heating coil welders were built either for use with a generator or with a mains adapter with an AC voltage input and output. Power control and consequently, the energy input control for the heating coil fitting, is usually regulated by phase angle control of the 230/50 Hz-voltage from the national grid, which is reduced to low-voltage output using a transformer. To regulate the electrical power input into the heating coil of the fitting at the outlet, the effective output voltage is measured, and via a control unit the phase angle or delay angle α of the phase angle cut-off is set. Idealized, the output signal from these heating coil welders resembles a phase angle controlled sinus wave with the fundamental frequency pulse of the input signal, i.e. 50 Hz in case of the above-mentioned mains voltage and between 40 Hz and 70 Hz for equivalent generator voltages, respectively.
After the allotted phase-out times, the since 1971 valid EU standard EN 61000-3-2 prohibits the use of the technique as described above for equipment with a mains power supply. Aside from other concerns, the standard is meant to especially restrict the feed of harmonic waves into the public low-voltage grid. The standards impose limit values for the impact on the public grid, such as for the phase shift between current and voltage due to inductive or capacitive loads, and harmonic waves due to the above-mentioned phase angle control. EN 61000-3-2 is equivalent to the German national standard, VDE 0838 “Electromagnetic Compatibility Limits for Harmonic Currents”.
The manufacturers of voltage or power supply devices have therefore stepped up the development of suitable voltage converter technologies. Power factor controllers (PFCs) are one solution to keep harmonic waves under the maximum value as set by the European standard EN 61000-3-2. In principle, there are two approaches, i.e. passive and active PFC. Passive PFC is based on the inductance in the user input circuit to temporarily store power from the grid, thus dampening power spikes. In active PFC, a device-integrated electronic control unit constantly monitors the user's power demand. Needed power is taken from the grid in almost perfect sinus configuration and temporarily stored. For their input circuits, these high-frequency voltage converters feature a cos(φ) very close to 1 and a rather small harmonic interference. The output from these units is DC voltage.
The use of DC voltage entails certain problems for implementations in the field of heating coil welding. Usually, the heating element for the fitting has the form of a coil and consequently, dependent on the coil geometry, the heating coil has an inductance. Since the prevailing heating coil welders work with an AC welding voltage, currently marketed heating coil fittings are indirectly designed and tested for use with AC welding voltage (50-60 Hz+/−15%) and the above-mentioned phase angle power control (phase angle cut-off). Therefore, the barcode of a fitting contains the effective welding voltage as a crucial parameter for welding operations with AC voltage in a defined frequency range.
The effective power PW,eff that is converted into heat in the heating coil fitting, i.e. the actual heat output or welding output to the heating coil, is calculated according to the following formula:
      P          w      ,      eff        =                              U          eff                ·                  I          eff                ·        cos            ⁢                          ⁢      φ        =                            U          eff          2                                                    R              2                        +                                          (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                  fL                                )                            2                                          ⁢              cos        ⁡                  (                      arctan            ⁡                          (                              2                ⁢                π                ⁢                                                                  ⁢                                  fL                  /                  R                                            )                                )                    
Ueff—effective value for the voltage on the heating coil fitting,
Ieff—effective value of the current through the heating coil fitting,
R—ohmic resistance of the heating coil fitting, and
L—inductance of the heating coil of the heating coil fitting
Due to its inductance L and in combination with its ohmic resistance R, the heating coil of the heating coil fitting constitutes a complex load R+i·2πfL, which causes a phase shift between current and voltage. The ratio of the heating coil reactance 2πfL and ohmic resistance R equals the tangent of the phase angle φ between current I and voltage U.
The formula above indicates that, given equal effective values for the DC and AC welding voltages on the fitting, the effective value for the active welding voltage in the heating coil fitting is always larger for DC voltage with a frequency of f=0 Hz than it is for the operation with AC welding voltage. It is equally apparent that the effective DC welding voltage decreases with increasing fundamental frequencies of the DC voltage.
Accordingly, the general formula for the welding power in the heating coil is:Pw,eff=f(U,R,L)
When the effective AC welding voltage is regulated via phase angle α of the phase angle control, the creation of harmonics must also be considered. Phase angle control leads to harmonics with frequencies that are usually multiples of the fundamental frequency. Therefore, the total welding power input into the heating coil consists of variable contributions from the fundamental wave and the harmonics. The higher the frequencies of the fundamental wave, the smaller are the amplitudes for the harmonics. In addition, the effect of the inductance L increases with increasing frequency. Increasing frequencies also reduce the harmonic current and with it the welding power contribution of the harmonic waves. The following formula for the heat output from the heating coil is slightly more precise:Pw,eff=f(U(α),R,L)
It must be noted that the voltage from the grid or from a generator, respectively, itself normally shows some variation. The phase angle α for the phase angle control of the desired effective DC welding voltage is therefore also not constant. This also subjects the welding power contribution from harmonics to deviations.
Finally, the ohmic resistance of the heating coil is temperature-dependent, i.e. the ohmic resistance of the heating coil increases with increasing temperature. In conclusion, the general formula for the effective welding power is therefore substitute specificationPw,eff=f(U(α),R(T),L)
where T stands for the temperature.
Test series have been run either with DC welding voltage or with AC welding voltage, in which the fundamental frequency clearly deviated from the fundamental frequency of the established DC welding voltage for the heating coil fitting. Experiments were done with DC voltages, which were equivalent to the effective AC welding voltage according to the barcode label on the fitting. The results revealed differences in actual welding power at the heating coil fitting of up to 50% for the worst case, particularly for pipes with large diameters.
In other words: When a heating coil fitting is designed for use with AC voltage and the established effective AC welding voltage Ueff in the barcode of the heating coil fitting is applied, then an equivalent DC welding voltage may deliver by 50% too much heat to the heating coil fitting. However, deviations larger than 5% can already compromise the quality of the welded joint. In experiments, this led to an overheating of the synthetic material and even to the collapse of pipes with thin walls. This condition is intolerable, especially for gas pipes with their high safety requirements.
Conversely, the welding energy input was decidedly lower when the fundamental frequency was higher than the frequency of the established DC welding voltage according to the fitting label. As a consequence, the welding process is executed with less than the predetermined welding energy input. This can be even more detrimental than the above-mentioned overheating of the welding spot because the inferior weld due to the less than optimal welding energy input may escape notice.
While the set time limits according to EN 61000-3-2 run out, an obvious and safe course of action would be to phase out all outdated heating coil welding equipment for use with AC voltage from the national grid and replace it with heating coil welding equipment that is designed for DC voltage input. For safety reasons, the use of fittings, which are not designed for use with DC welding voltage, should then also be prohibited. According to this scheme, the outdated heating coil fittings would have to either be discarded or returned to the manufacturer to be fitted with a new barcode. This would incur considerable costs.
In the global market for the heating coil welding technology, in the area of generator-supplied heating coil welders, as used in the exempt construction business, for example, there are however implementations, in which the supply voltage has a frequency of 200 Hz or higher. As a matter of principle, the use of fittings, which are not designed for such frequencies, would have to be prohibited too. However, this market is too small to offer small lots of heating coil fittings for such high fundamental frequencies, complete with labeling for the welding voltages and times—and then offer the fittings at the usual market prices for regular fittings.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.